1. Technical Field
The present invention relates to a detection circuit that is connectable to a magnetic sensor in which magnetoresistance elements (hereinafter also referred to as “MR elements”) are used and that detects a rotation angle of a magnetic field, and a semiconductor integrated circuit device. The invention also relates to a magnetic field rotation angle detection device including the magnetic sensor and the detection circuit. The invention further relates to an electronic device or the like, such as a water meter, a gas meter, or a speed meter, in which the magnetic field rotation angle detection device is used.
2. Related Art
MR elements have the property to change the value of their resistance depending on the intensity of a magnetic field because of the magnetoresistance effect. When a magnetic field is applied to an MR element, the value of its resistance increases. Thus, detection of the rotation angle of a rotating body that produces a rotating magnetic field has been performed by bridge-connecting a plurality of MR elements.
As a related technology, JP-A-2007-24738 (paragraphs 0011 to 0020 and FIG. 5) discloses a rotational angle detection device that enables rotational angle detection for rotational angles ranging from 0° to 360° with an extremely simple configuration. This rotational angle detection device includes a rotating magnet attached to a detection object and rotated with the detection object to generate a rotating magnetic field, a magnetic sensor positioned in a rotating magnetic field generated by the rotating magnet to position sensor units with a plurality of bridge-connected MR elements in such a way that their easy-magnetization axes are displaced by an angle of 45% and a four-pole auxiliary magnet positioned in the vicinity of the magnetic sensor to generate a synthesized rotating magnetic field ranging from 0° to 180° from a rotating magnetic field ranging from 0° to 360° provided by the rotating magnet in a region where the magnetic sensor is positioned.
According to JP-A-2007-24738 (paragraphs 0011 to 0020 and FIG. 5), since the four-pale auxiliary magnet is disposed in the vicinity of the magnetic sensor, a synthesized magnetic moment between, a synthesized magnetic moment of the four-pole auxiliary magnet and a magnetic moment obtained from the rotating magnet converts a rotational angle ranging from 0° to 360° provided by the rotating magnet into a rotational angle ranging from 0° to 180°. This enables a detected value for a rotational angle of between 0° and 180° at the magnetic sensor to be obtained as a detected value for a rotational angle between 0° and 360° for the detection object.
Moreover, with regard to a circuit for signal processing, known circuits can be used without change. A plurality of detection signals output from the magnetic sensor are subjected to A/D conversion in a plurality of A/D conversion circuits, respectively, and input to a CPU. The CPU performs predetermined signal processing of those detection signals and calculates rotational angle detection data.
JP-A-2007-24738 (paragraphs 0011 to 0020 and FIG. 5) is an example of related art.
However, use of a four-pole auxiliary magnet in a magnetic field rotation angle detection device significantly increases the component mounting area and the cost. Moreover, use of an A/D conversion circuit in a detection circuit significantly increases the current consumption of the circuit and the cost. Therefore, it has been desired to realize a high resolution without using a four-pole auxiliary magnet and an A/D conversion circuit. In that case, if the rotation angle of a magnetic field is detected using a magnetic sensor in which two sensor units each including a plurality of bridge-connected MR elements are arranged so as to be shifted by 45° from each other, the rotation angle can only be detected with a resolution of 45° (⅛ rotation).